In the semiconductor industry, the trend toward three-dimensional structures has been evident for several years now. Although a start was made with stacks of varying functional units more than ten years ago, the connections between these structures could only be produced in an expensive manner using outer wiring. This wiring was performed by wire bonders. The wiring is costly, susceptible to flaws and accordingly takes up a great deal of space.
For some years now, therefore, work has been underway on technologies in which the wiring of the varying functional units is carried out vertically by a substrate or by a chip. These passages are called vias. Vias in silicon are called TSVs (i.e., “through silicon vias”), and vias in polymer are called TPVs (i.e., “through polymer vias”).
All via technologies have in common the fact that they are produced in a basic substrate by drilling, milling or—preferably because of the very small dimensions in most cases—by etching. The smaller the vias, the more expensive the production technology. Since to date vias are already produced in the μm scale, virtually only drilling and etching techniques are taken into consideration.
Among drilling technologies, laser drills are the overwhelming choice. High-energy, extremely collimated laser beams produce such a high, locally greatly limited heat that vaporizes the basic material. By the laser drilling, vias with very large aspect ratios of diameter to depth can be provided. The laser drilling technology is used primarily in polymers. The vias through the polymers are referred to as “through-polymer-vias” or simply only as TPVs. Polymers can be thermally decomposed much more easily than metals or semiconductor materials.
Another type of production is the etching of vias. When using etching techniques, the surfaces of the substrates that are to be etched must first be masked and then treated with the respective chemicals.
After the production of vias, several possibilities of filling exist. On the one hand, only the surfaces of the vias can be coated with metal in order to thus produce a hollow conductor. On the other hand, the vias can be filled completely. The selection of the coating depends mainly on the use of the vias. Volume-filled vias are preferably better suited for higher current densities, while hollow conductor vias can be better used for other types of electronic loads.
A problem in the state of the art is that between the process step of the via production and the process step of the via coating or equipping, other process steps can be applied or must be applied to the substrate. These interposed process steps fairly often contaminate the vias that are produced. Although the contamination of the vias is done quite quickly, the corresponding and necessary cleaning is costly, time-consuming and fairly often incomplete. In the worst case, an incorrectly cleaned via can be coated incorrectly and thus leads to scrapping.
If, for example, a functional unit with 20 vias is connected to a second functional unit, the contamination of a single via is sufficient to make the entire functional unit unusable. Contamination of vias thus reduces the corresponding yield and raises unit prices.
Among all possible process steps that result in a contamination of vias, primarily the coating of the substrate surface with a temporary adhesive stands out. Temporary adhesives, so-called bonding adhesives, are polymers whose object is temporarily connecting to one another a product substrate, in particular a product wafer, to a carrier substrate, in particular a carrier wafer. Vias already present in the product substrate can be contaminated by temporary adhesives. After the product substrate was further processed and ultimately removed from the carrier substrate, the contaminated vias must be cleaned. The chemical properties of the temporary adhesive have an especially disadvantageous effect on the cleaning of the vias, since the temporary adhesive (primarily at low temperatures) has a high viscosity. The high viscosity of the temporary adhesive at room temperature, in particular at 20° C., is desired for the actual temporary bonding process in order to connect the product substrate firmly to the carrier substrate. In most debonding processes, the temperature must be elevated to remove the product substrate and the carrier substrate in order to reduce the viscosity of the temporary adhesive. Since the cleaning process, however, follows the debonding process and cannot be performed simultaneously, the viscosity in general increases again to a very high value, having a negative effect on the cleaning, by a dropping of the temperature between the debonding process and the cleaning process. In the case of debonding processes, which are performed at room temperature, the temporary bonding adhesive never reaches a low viscosity value.
In general, in order to be able to clean the vias, the product substrates must be put in a chemical bath after the debonding and be exposed in particular to ultrasound. Both treatment steps lay claim to the product substrate, in particular take time, and thus increase costs.